Epson SX600FW print quality problem?

Can't resolve a print quality horizontal banding issue. Have followed epson's manual and been on epson's chat line and discussed it with them (not helpful, unless you want to pay nearly the same price as the machine for them to repair it!). Nozzle check shows a .5 cm gap on lhs of bottom red line which hasn't cleared after numerous head cleans. Alignment is OK. Looks like I need to get inside it's workings but if I do, what am I looking for?